The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Understanding Body Metabolic Index in the UK
Body metabolic index (BMI) is a simple measurement used by the NHS and health professionals across the UK to assess whether your weight falls within a healthy range for your height. Unlike your standard weight alone, BMI takes into account your height, giving a more meaningful picture of your overall health status. This UK health guide explains what body metabolic index means, how it's calculated, and why the NHS uses it as a screening tool for weight-related health risks. It's important to understand that while BMI is useful, it has limitations and should always be considered alongside other health factors.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Body Metabolic Index Works in UK Healthcare
Body metabolic index (BMI) is calculated using a simple formula: your weight in kilograms divided by your height in metres squared. The NHS uses BMI as a screening tool to identify whether adults may be at increased risk of weight-related health conditions. In the UK, the standard BMI categories are: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30 and above).
However, the NHS recognises that ethnicity affects health risk at the same BMI level. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: healthy weight is 18.5–22.9, overweight starts at 23, and obesity at 27.5. This reflects research showing these groups face higher cardiovascular and diabetes risk at lower BMI values.
It's crucial to understand that BMI does not distinguish between muscle and fat. Athletes may have high BMI despite low body fat. BMI also doesn't account for where fat is stored, why your weight management journey matters, or individual health factors like medical conditions or medications.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
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Using BMI Alongside Other UK Health Measures
The NHS recommends using body metabolic index alongside waist circumference measurement, which is a better predictor of cardiovascular risk. Men should aim for a waist under 94cm (37 inches), with increased risk above 102cm (40 inches). Women should aim for under 80cm (31.5 inches), with increased risk above 88cm (34.5 inches).
Use our free BMI calculator above to check your personal BMI instantly and see where you fall within the NHS categories. If your BMI suggests you're overweight or obese, or if you have concerns about your health, your GP can provide personalised advice and discuss weight management options with you. Even a modest weight loss of 5–10% can bring significant health benefits including lower blood pressure, improved blood sugar control, and reduced joint strain.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
What's the difference between body metabolic index and BMI?
Body metabolic index and BMI are the same thing — the terms are used interchangeably in UK healthcare. It's simply the measure of weight relative to height, calculated as weight (kg) divided by height (m)². The NHS uses this single measurement as a screening tool for weight-related health risks, though it has limitations.
Why does the NHS use different BMI thresholds for different ethnic groups?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean heritage have higher risk of type 2 diabetes and heart disease at lower BMI levels. The NHS therefore recommends lower healthy weight thresholds for these groups — starting at BMI 18.5–22.9 rather than 18.5–24.9 — to better identify those at risk earlier.
Is a high body metabolic index always a sign of poor health?
Not always. BMI does not distinguish between muscle and fat, so muscular individuals (including athletes) may have high BMI but good health. BMI is a screening tool only and should be considered alongside waist circumference, fitness level, and overall health markers. Your GP can provide a full assessment.
Can I use body metabolic index to find my ideal weight range?
Yes. For a given height, the healthy weight range corresponds to a BMI of 18.5–24.9 (or 18.5–22.9 for certain ethnic groups). Use our free BMI calculator above to find your personal healthy weight range, then our weight converter to switch between kg, stone, and pounds as needed.
Should I aim for the lowest possible BMI?
No. The healthiest BMI range is typically 18.5–24.9 for most adults, or 18.5–22.9 for South Asian and other specific ethnic groups. Very low BMI can indicate underweight status, which carries its own health risks. Your GP can advise on the most appropriate target weight for your individual circumstances.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.